I have the same name so this is what I know. "Maah" in persian means moon and "wush" means face. Literally, it translates to face like the moon, I guess in terms of iconic beauty. This is further corroborrated by the fact that the name for having a face like a fairy in Persian is "Parivash" or "Pariwush." Ask anyone who is from Iran.
Mehwish has come to be popular because people from different regions have various accents and therefore differing pronunciations. "Meh" is wrong because in urdu/persian it means liquor/"sharaab" and "wish" is wrong because it becomes the hindi/sanskriti equivalent of "poison."